Halogen-catalyzed methane oxidation

Abstract

This paper highlights the importance of halogen-catalyzed methane oxidation in the upper troposphere and lower stratosphere. The calculated rate of methane oxidation is increased by at least 20% in the upper troposphere when halogen catalysis is included. In the lower stratosphere, approximately 25% of methane oxidation can be initiated by chlorine; the precise fraction is very temperature dependent. Including halogen-catalyzed methane oxidation increases the HOx and ClOx concentrations and decreases the NOx concentration. The calculated enhancement in the HOx concentration due to halogen-catalyzed methane oxidation is around 10-15% in the lower stratosphere; and around 20% in the upper troposphere. The decrease in the NOx concentration is around 10% in the upper troposphere. The enhancement in the ClOx concentration is around 7-10% in the lower stratosphere. The increase in the calculated HOx and ClOx concentrations and the decrease in the NOx concentration lead to a enhancement in the calculated O3 loss. The additional O3 loss calculated is most significant in the upper troposphere where over a 7-day simulation it was of the order of 0.1-1% for midlatitudes at equinox. As the atmospheric loading of chlorine drops the gross odd-oxygen production by NO + HO2 will increase, so there will be an accelerated ozone recovery. On a per molecule basis, bromine-catalyzed methane oxidation is approximately 2 orders of magnitude faster than chlorine catalyzed methane oxidation. In the upper troposphere bromine-catalyzed methane oxidation destroys ozone at a rate which is approximately one third of that at which nitrogen-catalyzed methane oxidation is producing ozone. Therefore, with the increasing atmospheric bromine loading, bromine-catalyzed methane oxidation is set to become more important. It would be valuable to have kinetic studies of the reaction BrO with CH3O2 so that the role of bromine-catalyzed methane oxidation can be quantified more precisely.